A novel combustion mode called premixed combustion (referred to hereinbelow as PCI combustion) has recently been suggested. In this system, fuel injection is carried out at a timing earlier than that for usual diesel combustion (referred to hereinbelow as diffusion combustion) and mixing of fuel and air is substantially enhanced to improve the engine exhaust gas. In the PCI combustion, exhaust gas recirculation (referred to hereinbelow as EGR) is performed with a high EGR ratio to obtain a low-oxygen atmosphere, thereby suppressing preignition and thus realizing low-NOx and low-SOOT (soot) combustion.
PCI combustion is typically limited to low-load operation regions. Accordingly, a combustion control method is known by which switching between the diffusion combustion and PCI combustion is performed according to the engine operation region.
For example, Japanese Patent Application Laid-open No. 2002-327638 suggests a diesel engine in which switching between PCI combustion and diffusion combustion is performed according to the operation region.
In the diesel engine described in Japanese Patent Application Laid-open No. 2002-327638, when switching is performed from diffusion combustion to PCI combustion, a control is performed to forcibly change the amount of air (amount of oxygen) so as to obtain an optimum amount of air for PCI combustion.